It is a great British debate which doesn't happen any longer. Bring back capital punishment? Almost from memory, Joe Public says yes. Our political parties, however, suck their thumbs and look away. Arguments are never joined and thus remain frozen, much as they were when the hangman's rope was shredded 40 years ago. But that doesn't mean there is no debate or that it has not moved on.

Scott Turow is famous for his deep-woven legal thrillers. He is also a distinguished criminal attorney, experienced both as prosecutor and defender, now partner in a great Chicago firm. He writes with elegant subtlety, brings formidable expertise to bear - and makes up his own mind. This brief book, sub-titled 'A Lawyer's Reflections on Dealing with the Death Penalty', is the fruit of all that: fresh, honest and memorable.

In March 2000, a tubby former drugstore chemist called George Ryan was governor of Illinois. He had a problem that troubled him rather more his fellow Republican, George W Bush. Ryan was a long-standing advocate of capital punishment, but he couldn't duck some grim figuring. Since 1977, when Illinois had restored the death penalty, 270 men and women had received the ultimate sentence. Yet only 12 of them, all appealings spent, had paid it. Thirteen had been completely exonerated on appeal. Ninety had been given life, or less. More than a third of the time that Illinois imposed a capital sentence, it seemed, the defendants were either not guilty or not deserving execution.

Surely some mistake here? Ryan did what politicians with a hot potato usually do: he set up an advisory commission. And Turow, a self-proclaimed 'agnostic' on the issue, was one of his nominees. In youth, he'd seen capital punishment as barbaric. But now, from his time at the prosecutor's table, he believed that some killers could be cruel, vicious and irredeemable, just 'bad people'.

'My job... was to make sure they didn't do bad things again. And I could see that a sentence of death was the most certain means to accomplish that goal in extreme circumstances.' Take Hector Reuben Sanchez, who randomly shot a boy in a Chicago car park then kidnapped, raped, sodomised and tortured his young girlfriend before beating her to death on a concrete floor. A bad man doing bad, bad things. His death sentence, Turow thought, was 'a victory' for the federal prosecutor's office. Practical justice.

And yet he'd been on the other side, too, handling pro bono the appeal of Alejandro Hernandez, sentenced to death for the kidnapping, rape and murder of a 10-year-old named Jeanine Nicarico. But when another man later confessed to the killing, the county prosecutors stubbornly refused for years to accept the man's confession, even after DNA evidence made him the killer and excluded Hernandez. This justice took years to achieve. This justice saw Hernandez eventually exonerated and several former prosecutors and policemen indicted on charges of conspiracy to obstruct it.

Where, Turow asked himself, was the shining principle in such dichotomies, the line which put Hernandez or another scared kid of a client, Chris Thomas, one one side and murderers like Sanchez or John Wayne Gacy, serial slaughterer of 33 young boys, on the other? Could you, indeed, find any principle here amid so much squalor and beastliness? And so, as the commission toiled, it was the practicalities which gnawed away at him.

Why did Texas, performer of more than a third of all US executions since 1976, still have a murder rate far above the national average? Why, within Illinois, were you five times more likely to get a death sentence for first-degree murder out in the rural sticks than you were in Chicago's Cook County? Why was capital punishment three-and-a-half times more likely when a man killed a woman? Why was the system so flaky, so corrupt, so reluctant to admit mistakes?

Why were the worst instances of incompetence reserved for murder cases when these were precisely the ones where most care ought to have been taken? Was it that you couldn't think again because death penalties, delivered in a red mist amid maximum public pressure, allowed no second thoughts? Wasn't this all politics?

The outcome of Ryan's commission, in such dislocated circumstances, seems politically perfect. Turow and friends did a smooth, standard job, refettling this, reforming that, bringing 'the system' up to date. But the governor, by then, had almost finished his term and wasn't running again. A free man. He wrestled with who to pardon and who to kill under the new rules - and gave up. He couldn't, he said, 'play God'. All 167 on Illinois's death row were saved. And Turow, by then, was a convert.

There will always be cases that cry out for ultimate punishment, but that is not the true issue. The pivotal question is whether a system of justice can be constructed that reaches only the rare, right cases without also occasionally condemning the innocent or underserving. Let Gacy live to save the Hernandezes and Thomases. Let justice be roughly done to save our politicians from posturing, pusillanimity and vote-grabbing. Let there be no more death by state decree.

The ultimate trouble is not that there is no final justice. We are too feeble and fallible to dispense it. The trouble is that we are human, too. Because Turow takes us on his own halting journey of self-discovery, we ride along with him: human beings grappling with a human dilemma. I finished reading it, by chance, just before George W's State of the Union address and felt cleaner for doing so.